How to Build E-Mail List?

Hi all,

I am currently writing my first novel, and while I realize I still have a long way to go, I have been reading around on here the past few weeks seeing what I will have to do once I am happy with my writing and eventual book cover. I have seen a lot of people writing about building an e-mail list to inform people about the release of your book. However, I have no idea how to even start that, which is why, as a someone new to the world of self publishing, I am here to ask the specific question of how to build a list of people who would be interested in the type of novel you are writing.

Basically that's it, I'm interested to see how that process works. For side notes, I am writing a sci-fi story which explores the origins of humans on Earth, so I would imagine there is a forum on here for sci-fi writers, which may be a good place to start to network with other writers.

Thank you for any replies/thoughts on this topic.